1. Utah contractor licensing and registration:

To verify Utah plumber or other contractor license use this online service.

Complaints:
You can file an online complaint on plumber, contractor, or electrician with the Utah Division of Occupational and Professional Licensing (DOPL) here. You may also download a complaint form and mail or fax it to:
Attn: Investigations
Division of Occupational & Professional Licensing
160 E 300 S
PO Box 146741
Salt Lake City, UT 84114-6741
Fax: (801) 530-6301
You can check if there were any disciplinary actions against your contractor here or using this online search.

You can complain about plumber or contractor to the Utah Division of Consumer Protection by filling out this complaint form and mailing it with the copies of supporting documents to:
Division of Consumer Protection
160 East 300 South, 2nd Floor
P.O. Box 146704
Salt Lake City, Utah 84114-6704

OR by fax to: (801) 530-6001

3. Better Business Bureau:

Go to http://www.bbb.org/ and enter your zip code. When your local BBB page appear click on the "FILE A COMPLAINT" link and follow directions.

You can file your Utah small claims suit in the justice court where your contractor lives, or where he performed the service for you.To start small claims lawsuit in Utah file your affidavit (complaint) with the clerk of a court. More information on filing.
At the time of filing you will have to pay a filing fee.
You can find where to file your Utah small claims suit here.

Maximum monetary limit of claims: $10,000 (including attorney fees);

Attorney: allowed, but not required;

Right to appeal: yes, within 30 days

You can learn more about Utah Small Claims and find all the forms here.

5. On-line complaints.

We collect all sorts of customer reviews, complaints, and comments here.
To post your comment on Google find your contractor in Google maps, and click the more info link next to the name of your contractor. On the following page, scroll down to Reviews by Google users and click on Rate and review link on the left.
There are many other websites that let you post comments and complaints about your contractor. Below you may find just a few examples:
1) http://www.ripoffreport.com (registration required),
2) http://www.complaintsboard.com/
3) www.complaints.com